In such an arrangement, the instrument panel body is provided with an aperture which is closed at its top face by a flap, and with a firing canal fixed on the underside face of the instrument panel body while surrounding the aperture.
The firing canal has an upper face which conforms to the curved shape of the instrument panel body, and an underside face which is generally flat and to which an airbag module of standardized type is fixed.
When the airbag inflates, it passes through the firing canal to come to press against the underside face of the flap, thus causing the flap to open. The airbag then passes through the aperture in the instrument panel body to deploy in the passenger compartment of the vehicle in order to protect the occupants thereof.
In general, the aperture has a rectangular outline and the flap is articulated around the transverse edge of this aperture which is furthest forward in relation to the direction of forward travel of the vehicle, i.e. the one closest to the windshield.
Given the speed with which the flap opens, the flap may shatter against the windshield during the opening brought about by the inflation of the airbag. This shattering may result in flap debris being thrown out in the direction of the occupants of the vehicle, this being liable to injure the occupants.
In order to overcome this disadvantage, document US2005127642 teaches the provision of lateral tethers securing each lateral edge of the flap to the corresponding lateral edge of the aperture, these tethers being slack when the flap is closed. The opening of the flap is therefore limited by the lateral retaining tethers which become taut when the airbag inflates, thus ensuring that the flap cannot strike the windshield.
The loads that arise at the time of opening are great because of the speed of deployment which is desirable for the airbag which means that the mechanical strength of the retaining tethers is generally insufficient. In practice, the tethers have a tendency to become torn out of the instrument panel body, damaging the flap and thereby creating a risk of debris being thrown out in the direction of the occupants of the vehicle.